Tyre carcasses are normally made on a first-stage building drum, which comprises two coaxial half-drums movable in opposite directions along a common longitudinal axis by an actuating device.
Each half-drum comprises an expandible bead carrier normally defined by a ring of sectors, which are movable, substantially radially with respect to the common longitudinal axis and by a normally pneumatic actuating device, to and from an expanded position clamping a respective bead bundle in a given axial position along the half-drum and radially outwards of a carcass body laid flat on the building drum.
Each half-drum also comprises at least one turn-up device normally defined by a bladder which, when idle, rests flat on the half-drum, axially outwards of the bead carrier, and is inflatable to turn an annular lateral portion of the carcass body up and outwards about the bead bundle.
This turn-up operation is normally performed either with the central portion of the carcass body between the bead bundles still flat (in which case, the half-drums are not needed), or after torus-shaping the central portion by moving the two half-drums towards each other, and feeding compressed air into the torus directly and/or with the interposition of bladders, which are inflated inside the torus to somehow counterbalance the outer axial thrust of the turn-up bladders.
In the first case, the turn-up operation involves turning over the longitudinal lateral portions of the carcass body 180°, thus inevitably exerting harmful twisting moments on the bead bundles.
In the second case, the turn-up operation is limited to a roughly 90-100° turn-up with substantially no twisting moments, but calls for a much more complicated drum featuring inner bladders and/or seals to inflate the central portion of the carcass body and form an annular shoulder axially inwards of each bead bundle.
In connection with the above, it should be pointed out that the carcass body in question is still green, and therefore unable to safely withstand the internal pressures needed to counterbalance the thrust of the turn-up bladders and ensure firm adhesion of the turned-up longitudinal lateral portions to the central portion of the carcass body.
To avoid having to feed compressed air into the carcass body to form the annular shoulders, building drums have been proposed—for example, in U.S. Pat. Nos. 3,414,446, 3,684,621, US-2008/264570, EP-0459728, U.S. Pat. No. 3,833,445, EP-0468580 and U.S. Pat. No. 4,149,927—featuring various types of deformable cages located between the bead carriers, and which expand radially outwards to support the central portion of the carcass mechanically from the inside and counterbalance the thrust of the turn-up bladders.
Though solving the problem of forming the annular shoulders and mechanically counterbalancing the thrust of the turn-up bladders, deformable cages of the type described seriously complicate the central part of the drum, which is normally quite simple in design, while the end portions of the drum, which are normally complicated in design, remain unchanged. In other words, known drums of the above type are so complex as to be unreliable.
Moreover, when expanded, the cages of known drums of the type described give rise to two separate structures or one axially weak structure. In other words, drums of this sort do not guarantee axially symmetrical tyres, and cannot be used anyway for producing relatively large tyres.